The mechanism of activation of antibody forming B cells in response to thymic independent and thymic dependent antigens has been investigated with regard to the requirement for cell interaction and to the definition of genes regulating such interactions. These questions have been approached in an in vitro antibody producing system that allows dissection of the contribution of T cells, B cells, and macrophages in primary antibody responses to TNP-Ficoll, TNP-LPS, TNP-Brucella abortus (all thymic independent) and TNP-KLH (thymic dependent). Genetic factors controlling such responsiveness have been investigated through the utilization of a mouse strain, CBA/N, which displays an X-linked immune deficiency to TNP-Ficoll and TNP-KLH antigens. Antibody responses to TNP-Brucella and TNP-LPS proceed in the absence of T cells or macrophages. The response to TNP-Ficoll is T cell independent but requires the participation of macrophages, and B cells. Distinct B cell populations respond to TNP-Brucella, TNP-LPS versus TNP-Ficoll and TNP-KLH, respectively. The immune defect of the CBA/N mouse is the result of the absence of a mature population of B cells which bear the cell surface marker Lyb5.